Bump nightly version

cbeuw · calebcartwright · commit 927561ace1ef · 2021-05-12T22:12:43.000-05:00
diff --git a/rust-toolchain b/rust-toolchain
@@ -1,3 +1,3 @@
 [toolchain]
-channel = "nightly-2021-05-04"
+channel = "nightly-2021-05-13"
 components = ["rustc-dev"]
